1. Field of the Invention
The present invention relates to the art of earth boring and the collection of formation fluid samples from a wellbore. More particularly, the invention relates to methods and apparatus for collecting a deep well formation sample and preserving the in situ constituency of the sample upon surface retrieval.
2. Description of Related Art
Earth formation fluids in a hydrocarbon producing well typically comprise a mixture of oil, gas, and water. The pressure, temperature and volume of formation fluids control the phase relation of these constituents. In a subsurface formation, high well fluid pressures often entrain gas within the oil above the bubble point pressure. When the pressure is reduced, the entrained or dissolved gaseous compounds separate from the liquid phase sample. The accurate measure of pressure, temperature, and formation fluid composition from a particular well affects the commercial interest in producing fluids available from the well. The data also provides information regarding procedures for maximizing the completion and production of the respective hydrocarbon reservoir.
Certain techniques analyze the well fluids downhole in the wellbore. U.S. Pat. No. 5,361,839 to Griffith et al. (1993) disclosed a transducer for generating an output representative of fluid sample characteristics downhole in a wellbore. U.S. Pat. No. 5,329,811 to Schultz et al. (1994) disclosed an apparatus and method for assessing pressure and volume data for a downhole well fluid sample.
Other techniques capture a well fluid sample for retrieval to the surface. U.S. Pat. No. 4,583,595 to Czenichow et al. (1986) disclosed a piston actuated mechanism for capturing a well fluid sample. U.S. Pat. No. 4,721,157 to Berzin (1988) disclosed a shifting valve sleeve for capturing a well fluid sample in a chamber. U.S. Pat. No. 4,766,955 to Petermann (1988) disclosed a piston engaged with a control valve for capturing a well fluid sample, and U.S. Pat. No. 4,903,765 to Zunkel (1990) disclosed a time delayed well fluid sampler. U.S. Pat. No. 5,009,100 to Gruber et al. (1991) disclosed a wireline sampler for collecting a well fluid sample from a selected wellbore depth, U.S. Pat. No. 5,240,072 to Schultz et al. (1993) disclosed a multiple sample annulus pressure responsive sampler for permitting well fluid sample collection at different time and depth intervals, and U.S. Pat. No. 5,322,120 to Be et al. (1994) disclosed an electrically actuated hydraulic system for collecting well fluid samples deep in a wellbore.
Temperature downhole in a deep wellbore often exceed 300 degrees F. When a hot formation fluid sample is retrieved to the surface at 70 degrees F., the resulting drop in temperature causes the formation fluid sample to contract. If the volume of the sample is unchanged, such contraction substantially reduces the sample pressure. A pressure drop changes in the situ formation fluid parameters, and can permit phase separation between liquids and gases entrained within the formation fluid sample. Phase separation significantly changes the formation fluid characteristics, and reduces the ability to evaluate the actual properties of the formation fluid.
To overcome this limitation, various techniques have been developed to maintain pressure of the formation fluid sample. U.S. Pat. No. 5,337,822 to Massie et al. (1994) pressurized a formation fluid sample with a hydraulically driven piston powered by a high pressure gas. Similarly, U.S. Pat. No. 5,662,166 to Shammai (1997) used a pressurized gas to charge the formation fluid sample. U.S. Pat. Nos. 5,303,775 (1994) and 5,377,755 (1995) to Michaels et al. disclosed a bi-directional, positive displacement pump for increasing the formation fluid sample pressure above the bubble point so that subsequent cooling did not reduce the fluid pressure below the bubble point.
Existing techniques for maintaining the sample formation pressure are limited by many factors. Pretension or compression springs are not suitable because the required compression forces require extremely large springs. Shear mechanisms are inflexible and do not easily permit multiple sample gathering at different locations within the wellbore. Gas charges can lead to explosive decompression of seals and sample contamination. Gas pressurization systems require complicated systems including tanks, valves and regulators which are expensive, occupy space in the narrow confines of a wellbore, and require maintenance and repair. Electrical or hydraulic pumps require surface control and have similar limitations.
Accordingly, there is a need for an improved system capable of compensating for hydrostatic wellbore pressure loss so that a formation fluid sample can be retrieved to the well surface at substantially the original formation pressure. The system should be reliable and should be capable of collecting the samples from the different locations within a wellbore.
The present invention provides an apparatus and method for controlling the pressure of a pressurized wellbore fluid sample collected downhole in an earth boring. The apparatus comprises a housing having a hollow interior. A compound piston within the housing interior defines a fluid sample chamber wherein the piston is moveable within the housing interior to selectively change the fluid sample chamber volume. The compound piston comprises an outer sleeve and an inner sleeve moveable relative to the outer sleeve. However, movement of the inner sleeve relative to the outer sleeve is unidirectional. An external pump extracts formation fluid for delivery under pressure into the fluid sample chamber. A positioned opened valve permits pressurized wellbore fluid to move said piston for pressurizing the fluid sample within the fluid sample chamber so that the fluid sample remains pressurized when the fluid sample is moved to the well surface.
The method of the invention is practiced by lowering a housing into a wellbore. The compound piston is displaced within the sample chamber by formation fluid delivered by the external pump. When the sample chamber has filled, a valve is opened to introduce wellbore fluid at hydrostatic wellbore pressure against the piston to move the piston for pressurizing the well fluid sample within the fluid sample chamber. By means of piston area differential, force on a inner sleeve of the compound piston is unbalanced to compress the fluid sample by a volumetric reduction. The reduced volume is secured by mechanically securing the relative positions of the compound piston against the sample chamber.